Urea dosing systems are typically used to reduce NOx emissions in large machines where space and weight considerations are not a concern, such as, for example, locomotives and engine-generators. Urea is a reductant that is typically mixed with water and stored in a tank that is located on the machine. As the machine operates and produces exhaust, the urea mixture is pumped from the tank into the exhaust system. The urea mixture may be mixed with air and sprayed into the exhaust system via a nozzle. The urea mixture may react with exhaust fumes at high temperatures, affecting a selective catalytic reduction (SCR) reaction that may reduce NOx emissions of the machine. A shortcoming of dosing systems relates to the relatively high ambient temperatures at which urea mixtures freeze, such as about −11° C. (about 12° F.). The urea mixture may freeze and expand within the dosing system, causing damage to intricate components such as injector nozzles. Additionally, when heated to intermediate temperatures between about 100° and 250° C., urea may decompose into a urea byproduct that may clog dosing components and/or an exhaust system.
U.S. Patent Application Publication No. 2007/0180816 A1 (the '816 publication) by Masuda et al. discloses an exhaust emission purifying apparatus for an engine. The '816 publication discloses a urea storage tank that is connected to an exhaust system via a passage. Urea is injected into the exhaust system via a nozzle located on the passage. The '816 publication also discloses a discharge-forcing device that forcibly discharges urea gas, which collects in an upper portion of the storage tank, to the exhaust system.
Although the '816 publication may provide a method for discharging urea gas from the upper portion of a storage tank, it may fail to purge urea from a nozzle that sprays urea into an exhaust system and thereby fail to prevent damage from frozen urea. The system of the '816 publication may also fail to remove urea from dosing components and/or an exhaust system at temperatures between about 100° and 250° C., which may allow the urea to decompose into ammonia polymer and cause clogging.
The present disclosure is directed to overcoming one or more of the shortcomings set forth above and/or other deficiencies in the existing technology.